The electroless nickel plating industry has long been involved in developing metal coatings for various substrates. These coatings are deposited on materials, both metallic and non-metallic, imparting the desirable physical and chemical properties of a nickel alloy to the surface. This electroless plating method typically employs reducing agents, such as hypophosphite, and is described generally as a controlled autocatalytic chemical reduction process for depositing the desired metal as a deposit or plating on a suitable substrate. The deposit is formed upon immersion of an appropriate substrate into an aqueous nickel plating solution in the presence of a reducing agent and under appropriate electroless nickel plating conditions. The electroless nickel alloy formed on the surface of the substrate is often referred to as a coating, film, deposit, or plated layer.
In the computer industry, hard disk data storage elements, or memory disks, are generally made from aluminum or an aluminum alloy substrate. Through any variety of processes, the substrate is treated or otherwise coated so that it may act as a repository for magnetic media which stores electronically written information onto the disk. Typically, electrolessly plating a nickel phosphorus alloy layer onto the bare aluminum or aluminum alloy substrate is undertaken to protect the substrate, providing a surface which is both chemically and mechanically appropriate for subsequent processing and deposition of magnetic media. Electroless nickel alloy plating of the substrate covers defects and provides a surface which is capable of being polished and super finished.
For memory disk plating applications, electroless nickel alloy plating is an established plating method which provides continuous deposition of a nickel phosphorus (NiP) alloy coating onto the memory disk substrate without the need for external electric plating current. The resulting NiP alloy coating is amorphous, and remains suitably non-crystalline upon subsequent annealing. The formation of nickel alloy crystallites in the coating would prevent the surface from being polished and super-finished to the standards required by the memory disk industry. One method of monitoring if NiP alloy crystallite formation has occurred in the coating is through magnetics measurements of the deposit. While the amorphous phase of the NiP alloy is nonmagnetic, the crystalline domains are magnetic.
As magnetic media technology evolves to higher areal density storage devices, the memory disk industry requires more robust characteristics of the electroless nickel alloy layer. One of these deposit characteristics is improved thermal stability, meaning the ability of the deposit to withstand exposure to higher annealing temperatures without crystallization. This inhibition of crystallization during annealing manifests itself as a suppression of the deposit's magnetization when compared to less stable materials. One way to achieve an increase in thermal stability of a nickel phosphorus alloy is through the incorporation of a suitable third component which aids in the inhibition of crystallization at elevated temperatures.
Inclusion of tin (Sn) in alloys where at least one constituent is nickel (Ni) has been accomplished previously by arc melting of bulk constituents and quench cooling the resulting mixture. These works lend evidence that adding Sn to a Ni alloy should help improve the thermal stability of that material. However, the arc melting process is not suitable for coating memory disk substrates industrially. Decomposition reactions have also been utilized to make Ni—Sn materials, but this method cannot produce a smooth, uniform coating and, as such, is not suitable for memory disk applications. Electroplating of Sn—Ni alloys is also known, but this method cannot produce a film with the flatness required for memory disk applications.
Nickel phosphorus tin (NiPSn) alloys have been made previously using electroless plating baths. However, these electroless deposition techniques typically used alkaline-based baths which utilized a stannate source for Sn, and were unable to achieve both greater than 3% Sn and 7-12% P in the deposited alloy. Often, alkaline-based baths also contain sulfur-based stabilizers/accelerators, like thiourea, which degrade the corrosion resistance properties of the deposit and prevent that bath's use for memory disk applications. Additional methods included the use of very acidic NiPSn baths, but were not found to be suitable for memory disk applications. In one case, a highly acidic bath was used (pH=0.5) which required high levels of tin and thiourea, and did not result in co-deposition of phosphorus, producing a crystalline deposit at unsuitably low deposition rates (˜0.6 μinches/minute). The crystalline nature of the deposit rendered it unsuitable for memory disk applications. In the other cases, the plating baths required a diboron ester, usually from glucoheptonic acid, or the formation of a stannate-gluconate complex in order to achieve co-deposition of tin. The plating baths in those works also required a greater amount of tin, and at pH<5 could not produce NiPSn deposits with both 3-9% Sn and 7-12% P under those conditions. In addition, some prior art plating baths utilized thiourea, which rendered the deposit unsuitable for memory disk applications.
Notwithstanding the prior art described herein, there is a need for an aqueous nickel phosphorus tin alloy electroless plating bath and process for chemically depositing that NiPSn alloy onto a memory disk substrate, wherein the deposited material is amorphous and possesses enhanced thermal stability as defined by the inhibition of crystallization and suppression of magnetization upon high temperature annealing. Though an obvious application for this type of aqueous nickel phosphorus tin alloy electroless plating bath and methodology for plating a substrate is in the memory disk industry, this bath and process could be used generally to apply a NiPSn alloy deposit to any appropriately activated material surface where a nickel alloy deposit is desired that possesses improved thermal stability.